1. Field of the Invention
The present invention relates generally to a method for producing a liquid crystal display and a method for cleaning a substrate.
2. Description of the Prior Art
Liquid crystal displays are thin and light and have low electric power consumption so as to be widely used for note-type and subnote-type personal computers. With the improvement of performance of personal computers, it has been required to increase the display capacity and display area of such liquid crystal displays and to improve the picture quality thereof. With the requirement of the improvement of picture quality, the deterioration of picture quality has been caused by slight scratch defects and chrominance non-uniformity produced during the manufacturing process. In order to prevent the occurrence of such scratch defects and chrominance non-uniformity, fine contaminations (particles) as well as organic films adhered to substrates must be removed in cleaning steps which are carried out as pretreatment steps in respective processes.
Referring to FIGS. 8 and 9, a conventional cleaning step for use in a method for producing a liquid crystal display will be described blow.
FIG. 8 shows a step of cleaning a substrate before forming an alignment layer, and FIG. 9 shows a step of cleaning a substrate after forming an alignment layer. When the substrate is e.g., an array substrate, i.e., a transparent substrate on which a plurality of scanning lines intersect a plurality of signal lines and wherein thin film transistors (each of which will be hereinafter referred to as a "TFT") serving as switching elements are arranged on the respective intersecting portions, some of the TFTs being connected to pixel electrodes, the term "substrate before forming the alignment layer" means a substrate on which pixel elements are formed. When the substrate is a counter substrate facing the array substrate, e.g., a color filter substrate (which will be also hereinafter referred to as a "CF substrate"), i.e., a transparent substrate, on which grid-like shading films of a black organic resin are arranged and a plurality of colors of colored layers of red, blue and green are arranged in the form of stripes so as to fill in the spaces between the grid and wherein a common electrode of ITO (indium tin oxide) is formed on the whole surface of the colored layers, the term "substrate before forming the alignment layer" means a substrate on which at least a common electrode is formed.
Referring to FIG. 8 again, a step of cleaning a substrate before forming an alignment layer will be described. First, such a substrate 2 is fed to a brush cleaning unit 120 by means of transfer rollers 111. Then, a cleaning solution is sprayed on the substrate 2 from nozzles 125. This cleaning solution is prepared by mixing a surface active agent with a pure water so as to have a predetermined concentration. The cleaning solution is prepared in a mixing tank 110 and pumped out by means of a pump 115 to be supplied to the nozzles 125.
The surface of the substrate 2, on which the cleaning solution has been sprayed, is rubbed by brushes 123, so that relatively large contaminations (particles) are removed from the substrate 2 to clean the substrate 2. Therefore, the waste water discharged from the brush cleaning unit 120 contains a surface active agent of high concentration.
Then, the substrate 2, which has been cleaned by the brushes 123, is fed to a pure water jet unit 130 by means of transfer rollers 111. Then, a high pressure pure water is sprayed on the substrate 2 from jet nozzles 135. Thus, the surface active agent on the substrate 2 is replaced with a pure water, and particles are removed by the high pressure pure water. Furthermore, the high pressure pure water is pumped out of a pure water storage tank (not shown) by means of a high pressure pump 133 to be supplied to the jet nozzles 135, and jetted from the jet nozzles 135 as a jet flow.
Then, the substrate 2 is fed to a shower unit 140 by means of transfer rollers 111. Then, a pure water is sprayed on the substrate 2 from shower nozzles 143 to wash the surface active agent and the water treated in the jet unit 130 (the water containing a surface active agent of intermediate concentration). Therefore, the waste water discharged from the shower unit 140 contains a surface active agent of low concentration.
Then, the substrate 2 is fed to an ultrasonic cleaning unit 150 by means of transfer rollers 111. A pure water, to which an ultrasonic vibrational energy is applied, is jetted out of an ultrasonic nozzle 154 driven by an oscillator 153 to be sprayed on the substrate 2. Thus, the relatively fine particles adhered to the substrate 2 are removed, and the water used in the shower unit 140 is washed out. Thereafter, the substrate 2 is fed to a shower unit 160 by means of transfer rollers 111, and a pure water is sprayed on the substrate 2 from shower nozzles 163, so that the water used for the cleaning in the ultrasonic cleaning unit 150 is washed out. Furthermore, the waste waters discharged from the ultrasonic cleaning unit 150 and the shower unit 160 can be treated as usual waste waters since the concentrations of the surface active agents contained in the waste wafers are very low.
Then, the substrate 2 is fed to a draining unit 170 by means of transfer rollers 111, and the water adhered to the surface of the substrate 2 is removed by air knives 172. Thereafter, the substrate 2 thus drained is transported to a hot plate 180 to be dried thereon.
Then, an alignment layer material is applied on the substrate 2 thus dried, and the rubbing thereon is carried out to form an alignment layer thereon. Referring to FIG. 9, a step of cleaning the substrate 2, on which the alignment layer has been formed, will be described. First, the substrate 2 is mounted in a bath 224 of an ultrasonic cleaning unit 220 by means of a loader (not shown). To this bath 224, a cleaning solution is supplied from a mixing tank 210 via a pump 215. This cleaning solution is prepared in the mixing tank 210 so as to contain a predetermined concentration of surface active agent. To the lower portion of the bath 224, an ultrasonic vibrational energy is applied by means of an oscillator 223. Therefore, the ultrasonic vibrational energy is applied to the substrate 2 in the cleaning solution to clean the substrate 2.
Then, the substrate 2 is fed to a pure water jet unit 230 by means of transfer rollers 211. Then, a high-pressure pure water is sprayed on the substrate 2 from jet nozzles 235 to replace the surface active agent with a pure water and to remove the particles on the substrate 2. The pure water is pumped out of a pure water storage tank (not shown) to be supplied to the jet nozzles 235. Furthermore, although the waste water discharged from the ultrasonic cleaning unit 220 contains a surface active agent of high concentration, and the waste water discharged from the pure water jet unit 230 contains a surface active agent of intermediate concentration.
Then, the substrate 2 is fed to a shower unit 240 by means of transfer rollers 211, and a pure water is sprayed on the substrate 2 from shower nozzles 243. Thus, the surface active agent and the water used in the jet unit 230 are washed out. The waste water discharged from the shower unit 240 contains a surface active agent of low concentration.
Then, the substrate 2 is fed to an ultrasonic cleaning unit 250 by means of transfer rollers 211. Then, a pure water, to which an ultrasonic vibrational energy has been applied, is jetted out of an ultrasonic nozzle 254 driven by an oscillator 253 to be sprayed on the substrate 2. Thus, the relatively fine particles adhered to the substrate 2 are removed, and the water used in the shower unit 240 is washed out.
Thereafter, the substrate 2 is fed to a shower unit 260 by means of transfer rollers 211. Then, a pure water is sprayed on the substrate 2 from shower nozzles 263, and the water used for the cleaning in the ultrasonic cleaning unit 250. Furthermore, the waste waters discharged from the ultrasonic cleaning unit 250 and the shower unit 260 can be treated as non-industrial waters since these waters contain a surface active agent of very low concentration.
Then, the substrate 2 is transported to a spin dry unit 270 to be fixed to a spin table 274. Since the spin table 274 is rotated by a motor 272, the substrate 2 is also rotated with the spin table 274, so that the water adhered to the surface of the substrate 2 is drained. Thereafter, the substrate 2 is transported to a hot plate 280 to be dried thereon.
As described above, in the conventional method for producing a liquid crystal display, the surface active agents are used as the cleaning solutions. These surface active agents are harmful to the natural world, and the waste waters discharged from the respective units contain different concentrations of surface active agents, so that it is required to separately recover and discharge the waste waters from the respective units. In addition, it is difficult to replace the surface active agents with water, and when the surface active agents remain on the substrate, the surface active agents form chrominance non-uniformity thereon to deteriorate the display quality, so that a great amount of pure water is required to rinse the substrate and it takes a lot of time for the rinsing. Therefore, there is a problem in that the manufacturing costs are increased.
Moreover, the cleaning solution containing the surface active agent must be diluted with a pure water so as to have a predetermined concentration in order to obtain a good detergency, and the temperature thereof must be adjusted so as to have a predetermined temperature, so that the there are problems in that the size of the system is increased and the manufacturing costs are increased.